In conventional pattern tracing systems of the optical type, a scanner mirror forms part of an optical system which normally projects a photocell onto a line or edge which is part of the pattern to be traced. When the photocell traverses the pattern, signals are obtained from the photocell which are used to derive directional and displacement signals which in turn are used to derive signals for X and Y coordinates servo-systems that maintain the optical scanning system positioned properly above the pattern while driving it along the pattern at a preselected speed.
The speed signals for the tracing head are generally set to a predetermined value on a control panel, which value when multiplied by the sine and cosine of the angle that the pattern makes with a reference direction form individual coordinate speed signals which are usually directly added to the servo-input signals. Sine and cosine signals are derived from the optical scanning information, either with electromechanical synchro-resolvers in the tracing head or with electronic sine/cosine resolving circuits, and these signals are fed to the respective servo-motors to control servo-motor speed along each coordinate so that the tracing head follows the pattern to be traced.
Optical pattern tracers of this type are widely used in the industry to guide machine tools such as flame cutting machines or milling machines along a path identical to that on a flat or two-dimensional pattern. Examples of these systems are illustrated in the Barry et al. U.S. Pat. No. 2,499,178, the Brouwer U.S. Pat. No. 3,017,552, and the Jewel U.S. Pat. No. 3,322,952.
In these optical tracing systems, pattern-line catching has always been a difficult problem and in fact many commercially available tracers have totally unreliable pattern-line catching systems. Still other pattern catching systems known in the prior art are capable of achieving adequate pattern catching only when the pattern is approached at a shallow angle by the tracing head. Frequently the pattern catching systems in known optical pattern tracers either fail to catch the line at all or are uncertain as to the direction of travel following catching. Most pattern catching systems presently known are incapable of distinguishing direction once the scanner is locked onto a line.
The present invention relates particularly to optical scanners that project a photocell in a circular or eliptical scan path on the pattern-line. They usually include an electronic resolver that consists of a sine/cosine generator for producing four sine waves mutually shifted in phase by 90 degrees. These sine and cosine waves are each fed to a sample and hold circuit, and a pulse train derived from signals from the photocells momentarily opens the sample and hold circuits at a given phase angle and causes capacitors in the sample and hold circuits to be charged at DC values corresponding to the instantaneous values at the phase angle of the cosine and sine waves from the generator. These DC voltages are fed to X and Y servo-amplifiers on a machine frame for the tracer to drive a compound slide in a direction corresponding to the phase of photocell signals.
In automatic steering systems the phase of the pulses from the photocell controls the opening of the sampling and hold circuits and such a photocell processing system is illustrated in my co-pending U.S. patent application Ser. No. 108,549 filed Dec. 31, 1979 now U.S. Pat. No. 4,371,782, entitled An Optical Pattern Tracing System and corresponding International application No. PCT/U.S. 80/01717, filed Dec. 23, 1980, and reference should be made thereto for a complete description of the scanner mechanism and the manner of processing the photocell signals for this purpose.
Some of these prior optical pattern tracing systems include a pattern approach and catching sub-system that enables an operator to select a manual steering direction for approaching the pattern to be followed. One such sub-apparatus includes a mechanical sine/cosine potentiometer that is connected to receive the sine waves produced by the electronic generator. The potentiometer includes a rotatably mounted slider that engages four equally spaced arcuate resistors which are arranged to produce a sine wave in the slider having a phase proportional to the angle of the slider on the potentiometer. The slider sine wave is fed through a high gain amplifier that produces a square wave pulse train that is used as the directional pulse train for the purpose of manual steering. The operator thus selects the appropriate approach direction by setting the angle on the potentiometer slider. These resistive type manual resolvers are very expensive and add significantly to the cost of the overall tracing system. Moreover, the metal-to-metal contact in these potentiometers creates wear which, after a relatively short time, requires replacement of the entire potentiometer.